This invention is concerned with enhancing the features of an optical image.
The enhancement of image features by filtering out appropriate spatial frequency components, which is known as spatial filtering, has been demonstrated to be a powerful tool for various image processing applications over the years. The technique consists of performing a spatial Fourier transformation of an incident image using a spherical lens, aligning an appropriate spatial filter at the Fourier plane, then performing an inverse Fourier transformation of the beam emerging from the filter. The Fourier transform of the Fourier transform of the image, as modified by the filtering operation, enhances certain features of the original image, as determined by the filter. A number of the disadvantages of this technique are associated with the fact that spatial frequency components are removed. First, the overall intensity of the processed image is weaker, since some of the components of the image are removed. Second, after one stage of such spatial filtering, further processing of the same image to enhance different features is often not possible, since the spatial frequency components required for the second stage enhancement may be removed in the first stage. Third, the processed image will contain only the features selected by the filter and will exclude the remainder of the information from the original image, which can impose difficulty in deciphering information from the processed image. For these reasons, spatial filtering has some serious limitations which make the prior art approaches to this technique undesirable for many applications.
A special subset of image enhancement, edge enhancement, is an important technique for image recognition. Image recognition techniques such as correlation and convolution require low cross-talk between images for high signal-to-noise results. To reduce the image cross-talk between a reference image and the background, the test image should first be edge enhanced. Edge enhancement in the optical domain can be a relatively simple and instantaneous process, requiring only a lens to Fourier transform an image and a small filter to block out the DC component of the spatial frequency components. For most images, however, much of the energy in the image resides in the DC component, so that blocking out this component causes the loss of a significant fraction of the energy present in the original image.